There is known a conventional winding switching method for allowing wide-range operations from a low-speed area to a high-speed area by switching armature windings of an AC (alternating current) motor.
The conventional winding switching method takes measures to prohibit winding switching, for example, in a situation where the number of rotations of the AC motor is suddenly changed, in order to reduce a shock by winding switching. However, such measures have a problem that efficiency is low and further sufficient torque cannot be obtained, for example, when continuous acceleration is performed from a low speed to a high speed.
In this regard, the present applicant has proposed an inverter device that can reduce a shock by winding switching even during acceleration or deceleration, which is disclosed in Japanese Laid-open Patent Publication No. 2010-22165. More specifically, when a voltage reference exceeds the maximum voltage value that can be output to an AC motor, the inverter device disclosed in Japanese Laid-open Patent Publication No. 2010-22165 calculates a d-axis current reference correction value that is a correction value of a d-axis current reference and performs a process for correcting the d-axis current reference by using the calculated d-axis current reference correction value.
At this time, the inverter device disclosed in Japanese Laid-open Patent Publication No. 2010-22165 previously computes a d-axis current reference correction value before winding switching and corrects a d-axis current reference by using the previously computed d-axis current reference correction value at the time of winding switching. As a result, because the calculation delay of the d-axis current reference correction value that is one of generation causes of a shock by winding switching can be removed, the inverter device can perform smooth winding switching even during acceleration or deceleration.
Furthermore, the inverter device disclosed in Japanese Laid-open Patent Publication No. 2010-22165 preliminarily computes a d-axis current reference correction value by using a torque reference, a source voltage value, and a rotational speed of the AC motor as parameters.
However, the inverter device disclosed in Japanese Laid-open Patent Publication No. 2010-22165 has room for improvement in that an influence by winding switching can be further reduced.
For example, the inverter device disclosed in Japanese Laid-open Patent Publication No. 2010-22165 uses the rotational speed of the AC motor as a parameter for determining the d-axis current reference correction value. However, the rotational speed of the AC motor comes under the influence of transient phenomena by winding switching. In other words, there is a possibility that a detection value of the rotational speed of the AC motor includes a noise component caused by transient phenomena by winding switching. When the d-axis current reference correction value is computed by using the detection value that comes under the influence of the transient phenomena, there is a possibility that the resolution time of the transient phenomena is prolonged.